A developing device for developing electrostatic latent images on an image carrier by using a two-component developer that includes a toner and a carrier is known in the art.
As is well known, the toner density of a two-component developer, namely the ratio of toner weight to the total weight of carrier and toner, is extremely important in terms of stabilizing image quality. The toner density of a two-component developer changes as the toner is consumed during development. In order to deal with this, the specification of Japanese Patent Application Laid-Open No. 2002-116615 discloses detecting the toner density of developer in a developing device at suitable times using a developer density controller (ATR, or Automatic Toner Replenisher) and holding the toner density constant by replenishing the developing device with toner in accordance with a change in the toner density, thereby maintaining the quality of the image.
Various schemes have been put to practical use in the toner density sensing means and density controller for the developer in the developing device.
For example, a developer density control apparatus known in the art has optical-type density sensing means disposed in close proximity to a developing sleeve or developer transport path within a developing device, causes light to strike developer carried on and transported by the developing sleeve, which serves as a developer carrier, or developer contained in a developer vessel, senses the toner density of the developer by utilizing the fact that the reflectivity of the light reflected from the toner differs depending upon the toner density, and controls the toner density accordingly. Alternatively, use is made of an inductance-sensing-type developer density control apparatus (inductance ATR) that has an inductance head, which senses apparent magnetic permeability based upon the mixing ratio of magnetic carrier and non-magnetic toner, disposed on the side wall of a developing device, senses the toner density of the developer based upon a change in magnetic permeability, and replenishes toner based upon a comparison between the sensed toner density and a reference value.
In another method available in the art, a patch image for sensing density is formed on a photosensitive drum serving as an image carrier, the density of the patch is sensed by an optical sensor provided at a position opposing the surface of a photosensitive drum, the analog output obtained is converted to a digital signal and the digital signal is sent to a CPU. If the sensed density is greater than an initially set value, the CPU halts the replenishment of toner until the sensed density returns to the initially set value. If the sensed density is less than the initially set value, then the CPU replenishes the developing device with toner until the initially set value is restored, thereby regulating the toner density of the developer to a desired value.
In accordance with the density control method used in the inductance ATR, assume by way of example that the apparent magnetic permeability is sensed to be high. This means that the proportion of carrier contained in developer of fixed volume is large and that the toner density of the developer is low. In such case, therefore, the developing device starts being replenished with toner. Conversely, if the apparent magnetic permeability of the developer is low, this means that the proportion of carrier contained in developer of fixed volume is small and that the toner density of the developer is high. Accordingly, the developing device stops being replenished with toner. The toner density of the developer is thus controlled.
FIG. 1 is a sectional view illustrating a two-component developing device that employs such an inductance ATR. As shown in FIG. 1, a developing device 50 has a developing vessel 51 containing a developer. A hollow metal sleeve (developing sleeve) 52 serving as a developer carrier is freely rotatably disposed in an opening of the developing vessel 51 in close proximity to a photosensitive drum 1. A magnet roller 53 of magnetic-field generating means is disposed non-rotatably within the developing sleeve 52. A blade 54 for regulating the thickness of a toner layer is provided below the developing sleeve 52 in close proximity to the sleeve. Developer transported with rotation of the developing sleeve 52 in the direction of the arrow has its layer thickness reduced by the blade 54.
Placed inside the developing vessel 51 substantially in parallel with the developing sleeve 52 are a first screw (A screw) 55 and a second screw (B screw) 56 on the side of a partitioning wall 57 opposite the A screw 55. The A and B screws 55 and 56 rotate in the directions of the arrows and serve to stir and transport the developer in mutually opposing directions along the longitudinal direction of the screws. The partitioning wall 57 is not connected to the wall of the developing vessel 51 at both ends along the longitudinal direction, and openings therefore are defined between the partitioning wall 57 and the wall of the vessel. Developer that has been transported by the A screw 55 is delivered to the side of the B screw 56 through one of the openings in the partitioning wall 57, and developer that has been transported by the B screw 56 is delivered to the side of the A screw 55 through the other opening in the partitioning wall 57. As a result, the developer is made to circulate between the A and B screws 55 and 56, respectively.
Developer that has been used in development on the side of the A screw 55 is sent to the side of the B screw 56 and the toner density is sensed by a toner density sensor 58 situated upstream of the B screw 56 in terms of the direction of developer transport. A suitable amount of toner is supplied from a toner accommodating unit 60 through a replenishing cylinder 59 downstream of the toner density sensor 58 in such a manner that the output value of the toner density sensor 58 will become a value (referred to as a “target value” below) corresponding to a desired toner density.
The toner accommodating unit 60 is removably mounted on the main body of an image forming apparatus. FIG. 2 illustrates the structure of the toner accommodating unit 60. A toner replenishing port 61 is connected to the replenishing cylinder 59 from which the toner is supplied by rotation of a toner replenishing screw 62. In a case where it has been determined from the result of detection by the toner density sensor 58 that the toner density is low, a signal is sent to the CPU of the image forming apparatus in such a manner that additional toner is supplied. Upon receiving this signal, the CPU performs a replenishing operation by rotating a drive motor that drives the toner replenishing screw 62. Since the relationship between the rotational speed of the screw and the amount of toner replenishment is known in advance, the rotational speed of the toner replenishing screw 62 is decided based upon the result from the toner density sensor 58, a desired amount of toner is supplied and the toner density of the developer is held constant at all times.
Absence of toner in the toner accommodating unit 60 of the developing device 50 is sensed in the manner described below. FIG. 3 illustrates a change in the output value of the toner density sensor 58.
First, when toner is consumed, the ratio of carrier increases and therefore the output value of the toner density sensor 58 rises. Toner replenishment from the toner accommodating unit 60 is controlled in such a manner that the output value will become the target values, as described above. Of course, if there is toner inside the toner accommodating unit 60, then the output value of the toner density sensor 58 will approximately agree with the target value (the “toner-present state” in FIG. 3).
If toner runs out with use of the toner accommodating unit 60, the printing operation continues, toner is consumed and replenishment of toner ceases. As a result, the output value of the toner density sensor 58 gradually rises (the “no-toner state” in FIG. 3).
If a no-toner decision value is determined and the output value of the toner density sensor 58 attains this decision value, absence of toner is finally determined (the “final no-toner point” in FIG. 3) and the user is so informed as by presenting a display on the panel of a printer.
If the user has been notified of absence of toner, the toner accommodating unit is exchanged for a new one. A toner density restoration sequence usually is provided for the purpose of raising the density of toner in the developing device 50 so as to obtain a desired image at the time of the exchange, and for the purpose of checking that toner is contained in the replaced toner accommodating unit.
In the toner density restoration sequence, replenishment of toner is carried out in such a manner that the density of toner in the developing device 50 will become the desired toner density, i.e., in such a manner that the output value of the toner density sensor 58 will become the target value. When the output value of the toner density sensor 58 falls below the target value, the display indicating absence of toner is cancelled.
By exercising control in this fashion, the user is notified of the fact that the developing device 50 is in a usable state. In addition, it is possible to inform the user of the fact that the toner accommodating unit is normal.
Further, if the user has inserted a toner accommodating unit that is in the no-toner state or in a state in which a malfunction has occurred and toner cannot be supplied, toner density will not be restored. It is so arranged, therefore, that notification of absence of toner will not be cancelled in this case. Here an upper limit is decided for the time of the toner density restoration sequence, whereby the state of the toner accommodating unit can be judged after a prescribed length of time. This makes it possible to notify the user of the fact that the toner accommodating unit has no toner or of the fact that a malfunction has occurred.
However, if a user installs a new toner accommodating unit in an image forming apparatus and the toner density restoration sequence begins, the following problem arises with the above-described control method.
Specifically, regardless of the fact that a normal toner accommodating unit has been installed, there are instances where toner density does not manage to recover and the toner density restoration sequence continues for a long period of time.
This problem arises in the case of a so-called “packed state”, namely a state in which toner bulk diminishes and density rises when a toner accommodating unit has been left standing for a long period of time. If the packed state occurs, the toner will not be readily transported, even though the toner replenishing screw rotates, and the toner will not readily drop from the replenishing port. The toner will have even more difficulty dropping if the user has left the toner accommodating unit standing with its replenishing port facing upward. In other words, in a case where the user has extracted a toner accommodating unit and left it standing with the replenishing port facing upward, the toner replenishing screw will be at the top owing to the fact that the replenishing port is facing upward and therefore toner will not be present in the vicinity of the toner replenishing screw. In some cases the toner within the vessel will solidify. If this occurs, the desired amount of toner for replenishment will not be supplied even though the toner accommodating unit has been installed in the image forming apparatus and the toner replenishing screw is rotated.
Thus, in a case where a normal toner accommodating unit containing toner has been installed, it is necessary to raise the upper-limit time of the toner restoration sequence in such a manner that an erroneous decision to the effect that the toner accommodating unit is devoid of toner will not be rendered.
Since the upper-limit time of the toner restoration sequence is raised in this fashion, the time it takes to determine that a toner accommodating unit is devoid of toner will be longer than necessary in a case where this toner accommodating unit devoid of toner has been installed accidentally by the user. This is undesirable.